1. Field of the Invention
The present invention relates to a valve operating mechanism of the SOHC (single overhead camshaft) type for an internal combustion engine.
2. Description of the Relevant Art
There is known an SOHC valve operating mechanism in an internal combustion engine having cylinder bores each associated with a combustion chamber thereabove which has an ignition spark plug. Two intake valves are disposed in one side of each combustion chamber, and two exhaust valves are disposed in the other side of the combustion chamber. The valve operating mechanism includes a single camshaft extending above the cylinder head and positioned at the upper ends of the intake or exhaust valves, a rocker arm shaft disposed between the camshaft and the other valves, and rocker arms held in engagement with the camshaft and swingable about the rocker arm shaft. The upper ends of the intake or exhaust valves are engaged by the camshaft so that they can be opened and closed when the camshaft is rotated about its own axis. The upper ends of the other valves are engaged by the rocker arms and openable and closable by swinging movement of the rocker arms caused by rotation of the camshaft. The valve operating mechanism of this design is disclosed in Japanese Laid-Open Patent Publication No. 56-143322, for example.
For suppressing knocking and increasing combustion efficiency in the internal combustion engine equipped with the valve operating mechanism of the 4-valve SOHC type, it is desirable to position the spark plug at the center of the combustion chamber. However, if the spark plug were located centrally in the combustion chamber, the width and height of the cylinder head would be of increased values to prevent the spark plug from interfering with components of the valve operating mechanism upon insertion of the plug into the combustion chamber. Use of a member having a plug insertion hole on the cylinder head would result in an increased number of parts required for making up the valve operating mechanism.